Magnetic field suppression system

Abstract

Apparatus and methods to reduce unwanted magnetic fields and unwanted motion in precision instruments are described. A coil assembly that is used to generate an opposing magnetic field can include a first coil configured to generate a static magnetic field and a second coil configured to generate a time-varying magnetic field. The first and second coils can be in close proximity and sized to suppress magnetic fields over a large localized region. The first coil can be connected to a choke to increase its impedance seen by the second coil.

What is claimed is: 
     
       1. A magnetic field suppression system comprising:
 a first coil assembly having a first winding configured to generate a static magnetic field along a first direction through a center of the first coil assembly; and 
 a second winding configured to generate a time-varying magnetic field along essentially the first direction; 
 a second coil assembly oriented parallel to and spaced apart from the first coil assembly and having a third winding configured to generate a static magnetic field essentially along the first direction through a center of the second coil assembly; and 
 a fourth winding configured to generate a time-varying magnetic field essentially along the first direction. 
 
     
     
       2. The magnetic field suppression system of  claim 1 , further comprising a choke connected to the first winding. 
     
     
       3. The magnetic field suppression system of  claim 2 , wherein the choke is connected as a common mode choke. 
     
     
       4. The magnetic field suppression system of  claim 1 , wherein the first winding is located within 8 cm from the second winding. 
     
     
       5. The magnetic field suppression system of  claim 1 , wherein a maximum amount of current provided to the first winding by the magnetic field suppression system is greater than a maximum amount of current provided to the second winding. 
     
     
       6. The magnetic field suppression system of  claim 1 , wherein a minimum span of the first coil assembly is between 0.25 m and 10 m. 
     
     
       7. The magnetic field suppression system of  claim 1 , further comprising:
 DC control circuitry connected to the first winding and configured to provide a DC or slowly varying current having cyclic variations of 1 Hz or less to the first winding; and 
 AC control circuitry connected to the second winding and configured to provide at least a time-varying current having cyclic variations greater than 1 Hz to the second winding. 
 
     
     
       8. The magnetic field suppression system of  claim 7 , further comprising a magnetic field sensor arranged to sense magnetic field in a region in which the time-varying magnetic field is produced and to provide a signal indicative of the magnetic field to the AC control circuitry. 
     
     
       9. The magnetic field suppression system of  claim 1 , wherein a central region between the first coil assembly and the second coil assembly excluding conductive and ferromagnetic objects and spanning approximately one-third of a distance between the first coil assembly and the second coil assembly exhibits a magnetic field produced by the first coil assembly and the second coil assembly that is spatially uniform to within 10 percent. 
     
     
       10. The magnetic field suppression system of  claim 1 , further comprising a vibration-isolation platform, wherein the first coil assembly is configured to reduce magnetic field in a localized region and the vibration-isolation platform is configured to support a payload within the localized region. 
     
     
       11. A method of suppressing magnetic field in a localized region, the method comprising:
 applying DC current to a first winding of a coil assembly to generate a first magnetic field that reduces a static component of a magnetic field present in the localized region; 
 applying a first time-varying current to a second winding of the coil assembly to generate a second magnetic field that reduces a time-varying component of the magnetic field present in the localized region, wherein the first winding is located within 8 cm of the second winding; and 
 supporting, with a vibration-isolation system, a payload in the localized region. 
 
     
     
       12. The method of  claim 11 , further comprising attenuating a second time-varying current induced in the first winding by the first time-varying current with a choke connected to the first winding. 
     
     
       13. A system comprising:
 a first coil assembly having a first winding and a second winding, wherein:
 the first winding is configured to generate a static magnetic field along a first direction through a center of the first coil assembly, and 
 the second winding is configured to generate a time-varying magnetic field along essentially the first direction; and 
 
 a vibration-isolation platform, wherein the first coil assembly is configured to reduce a total amount of magnetic field in a localized region by suppressing ambient magnetic field in the localized region and the vibration-isolation platform is configured to support a payload within the localized region. 
 
     
     
       14. The system of  claim 13 , further comprising a choke connected to the first winding. 
     
     
       15. The system of  claim 14 , wherein the choke is connected as a common mode choke. 
     
     
       16. The system of  claim 13 , wherein the first winding is located within 8 cm from the second winding. 
     
     
       17. The system of  claim 13 , wherein a maximum amount of current provided to the first winding by the system is greater than a maximum amount of current provided to the second winding. 
     
     
       18. The system of  claim 13 , wherein a minimum span of the first coil assembly is between 0.25 m and 10 m. 
     
     
       19. The system of  claim 13 , further comprising:
 DC control circuitry connected to the first winding and configured to provide a DC or slowly varying current having cyclic variations of 1 Hz or less to the first winding; and 
 AC control circuitry connected to the second winding and configured to provide at least a time-varying current having cyclic variations greater than 1 Hz to the second winding. 
 
     
     
       20. The system of  claim 19 , further comprising a magnetic field sensor arranged to sense magnetic field in a region in which the time-varying magnetic field is produced and to provide a signal indicative of the magnetic field to the AC control circuitry. 
     
     
       21. The system of  claim 13 , further comprising:
 a second coil assembly oriented parallel to and spaced apart from the first coil assembly and having a third winding configured to generate a static magnetic field essentially along the first direction through a center of the second coil assembly; and 
 a fourth winding configured to generate a time-varying magnetic field essentially along the first direction, wherein a central region between the first coil assembly and the second coil assembly excluding conductive and ferromagnetic objects and spanning approximately one-third of a distance between the first coil assembly and the second coil assembly exhibits a magnetic field produced by the first coil assembly and the second coil assembly that is spatially uniform to within 10 percent. 
 
     
     
       22. A magnetic field suppression system comprising:
 a first coil assembly having a first winding configured to generate a static magnetic field along a first direction through a center of the first coil assembly; and 
 a second winding configured to generate a time-varying magnetic field along essentially the first direction; 
 DC control circuitry connected to the first winding and configured to provide a DC or slowly varying current having cyclic variations of 1 Hz or less to the first winding; and 
 AC control circuitry connected to the second winding and configured to provide at least a time-varying current having cyclic variations greater than 1 Hz to the second winding; and 
 a magnetic field sensor arranged to sense magnetic field in a region in which the time-varying magnetic field is produced and to provide a signal indicative of the magnetic field to the AC control circuitry. 
 
     
     
       23. The magnetic field suppression system of  claim 22 , further comprising a choke connected to the first winding. 
     
     
       24. The magnetic field suppression system of  claim 23 , wherein the choke is connected as a common mode choke. 
     
     
       25. The magnetic field suppression system of  claim 22 , wherein the first winding is located within 8 cm from the second winding. 
     
     
       26. The magnetic field suppression system of  claim 22 , wherein a maximum amount of current provided to the first winding by the magnetic field suppression system is greater than a maximum amount of current provided to the second winding. 
     
     
       27. The magnetic field suppression system of  claim 22 , wherein a minimum span of the first coil assembly is between 0.25 m and 10 m. 
     
     
       28. The magnetic field suppression system of  claim 22 , further comprising:
 a second coil assembly oriented parallel to and spaced apart from the first coil assembly and having a third winding configured to generate a static magnetic field essentially along the first direction through a center of the second coil assembly; and 
 a fourth winding configured to generate a time-varying magnetic field essentially along the first direction, wherein a central region between the first coil assembly and the second coil assembly excluding conductive and ferromagnetic objects and spanning approximately one-third of a distance between the first coil assembly and the second coil assembly exhibits a magnetic field produced by the first coil assembly and the second coil assembly that is spatially uniform to within 10 percent.